1. Field of the Invention
The present invention relates to a magnetic tunnel device, magnetic memory device, magnetoresistance effect head, and magnetic storage system.
2. Description of the Related Art
A magnetic substance of certain kind changes in electric resistance when it is placed in a magnetic field. This phenomenon is referred to as magnetoresistance effect. This effect is utilized for magnetoresistance effect elements (MR elements), such as magnetic head and magnetic sensor, in which the magnetic substance is in the form of magnetic layer. New magnetic memory based on MR elements has been proposed. (It is referred to as magnetic random access memory (MRAM).) Such MR elements are required to be highly sensitive to an external magnetic field and have a high speed of response.
MR elements with a ferromagnetic substance are characterized by good temperature stability and broad range of operating temperature. They are conventionally made with thin film of ferromagnetic alloy such as NiFe alloy. Unfortunately, it does not afford magnetic heads with a sufficient sensitivity because it changes only a little (2 to 3%) in magnetoresistance.
The magnetoresistance effect also manifests itself in a metal laminate film composed of magnetic layers and non-magnetic layers (several nanometers thick) laminated alternately. In this case, it is referred to as the great magnetoresistance effect (GMR), which is due to conduction electrons scattering depending on the direction of spin of the magnetic layer. GMR is attracting attention. For example, it has been reported that GMR is observed in Fe/Cr artificial superlattice film (Phys. Rev. Lett., 61, 2472 (1988)) and Co/Cu artificial superlattice film (J. Mag. Mag. Matter., 94, L1 (1991)). Unfortunately, such metal artificial superlattice film, with its magnetic layers bonded by antiferromagnetism, has a great antiferromagnetic exchange coupling constant and hence needs a strong magnetic field for saturation and suffers a great hysteresis.
There has been developed a new laminate film composed of two ferromagnetic layers and one nonmagnetic layer interposed between them. This film, referred to as spin-valve film, is constructed such that the nonmagnetic layer is thick, one ferromagnetic layer has its magnetization pinned, and the other ferromagnetic layer is readily magnetized (with spin reversal) by the external magnetic field. Unfortunately, the spin-valve film is low in resistance and hence low in output voltage; therefore, for the spin-valve film to produce a high output voltage, it is necessary that the sense current should be large. The result in the case of magnetic head with spin-valve film is that magnetization in the magnetization-pinned layer is reversed by electrostatic destruction (ESD).
The above-mentioned multilayer film (such as artificial metal superlattice film) greatly changes in magnetoresistance when current passes through it in the direction perpendicular to the film surface, as reported in Phys. Rev. Lett., 66, 3060 (1991). (This phenomenon is called the perpendicular magnetoresistance effect.) However, this effect cannot be measured at room temperature without microfabrication of submicron order because the current path is small and each metal layer has a low resistance.
The GMR effect due to spin-dependent conduction is found in the granular film which is composed of a non-magnetic metal matrix and magnetic superfine particles dispersed therein, as reported in Phys. Rev. Lett., 68, 3745 (1992). In the absence of magnetic field, this granular film has a high electric resistance because individual magnetic superfine particles have their spin oriented in irregular directions. In the presence of magnetic field, they have their spin aligned with the direction of magnetic field, decreasing in resistance. This produces the spin-dependent magnetoresistance effect. However, the magnetic field for saturation in this case is inherently strong because the magnetic superfine particles exhibit the superparamagnetism.
On the other hand, there has been found another great magnetoresistance effect which results from the ferromagnetic tunnel effect rather than the spin-dependent scattering. This effect manifests itself in a laminate film with tunnel junctions, which is composed of two ferromagnetic metal layers and one dielectric layer interposed between them, when current flows in the direction perpendicular to the film surface such that tunnel current occurs in the dielectric layer. The great magnetoresistance effect is due to the fact that when spin reversal takes place in the ferromagnetic metal layer with a small coercive force, the tunnel current greatly varies depending on whether spins in the two ferromagnetic metal layers are parallel or antiparallel to each other. This is known to stem from the spin asymmetry of state density in the Fermi surface.
The ferromagnetic tunnel junction element as mentioned above changes in magnetoresistance rather greatly but suffers the disadvantage that it increases in resistance to 1 to 10 Mxcexa9 when it is in the form of microfabricated element of the order of several micrometers squared. This high resistance leads to low response speeds and large noises.
There has been proposed a ferromagnetic tunnel junction which utilizes cobalt fine particles (2 to 4 nm in diameter) dispersed in a dielectric material. (Phys. Rev., B56(10), R5747 (1997)) Unfortunately, such cobalt fine particles exhibit superparamagnetism and inherently need a strong magnetic field for saturation like the granular film mentioned above. Moreover, the ferromagnetic tunnel junction element using cobalt fine particles changes in magnetoresistance only half as much as the one using a dielectric layer.
It is theoretically predicted that the double tunnel junction constructed of Fe/Ge/Fe/Ge/ferromagnetic material produces significant magnetoresistance owing to the spin-polarized resonance tunnel effect. (Phys. Rev., B56, 5484 (1997)) This prediction, however, is about behavior at an extremely low temperature (say, 8 K) and no predictions have been made about behavior at room temperature. Moreover, nothing has been reported about actual production of double tunnel junction.
Among other tunnel effect elements than mentioned above is a ferromagnetic tunnel effect element, which has been applied for patent (U.S. patent application Ser. No. 09/074,588). It is composed of a granular magnetic film and two electrodes arranged in the proximity thereof, the former being made of ferromagnetic fine powder (with coercive force) dispersed in a non-magnetic dielectric matrix, at least either of the latter being made of a ferromagnetic material. The advantage of this element is that the granular magnetic film is so thick (tens of nanometers) that it does not greatly fluctuate in magnetoresistance but greatly changes in magnetoresistance even in a small magnetic field. However, the fact that the element has two tunnel barriers at the boundaries between the two electrodes and the particles in the granular magnetic film leads to the disadvantage that the two tunnel barriers would fluctuate. In addition, it is likely that the ferromagnetic particles in the granular magnetic film have a small coercive force if they are small in size.
It is an object of the present invention to provide a new magnetic element which is different from the above-mentioned tunnel effect element. It is another object of the present invention to provide a magnetic element which readily and stably exhibits a large rate of change of magnetoresistance in a small magnetic field and fluctuates only little in resistance and sensitivity to magnetic field. It is another object of the present invention to provide a magnetic device, such as magnetic head and magnetic memory element, that is based on said magnetic element.
The first aspect of the present invention is a magnetic element. This element is constructed such that tunnel current flows between a ferromagnetic layer and a ferromagnetic-dielectric mixed layer and the magnetic layer with a smaller coercive force has its spin switched, thereby producing the magnetoresistance effect.
The magnetic element according to the first aspect of the present invention should preferably be constructed such that the dielectric material in the ferromagnetic-dielectric mixed layer is dispersed in granular form in the matrix of ferromagnetic material.
The magnetic element of the present invention has a laminate film composed of a ferromagnetic-dielectric mixed layer and a layer of dielectric material, and the layer arrangement of the laminate film may be represented as follows:
A/(B/A)N (where Nxe2x89xa71, denoting the number of layers)
A: the layer of dielectric material
B: the ferromagnetic-dielectric mixed layer.
In the vicinity of this laminate film is arranged a ferromagnetic layer. A few examples of the laminate structure are given below.
A laminate film of A/(B/A)N structure in combination with a ferromagnetic layer.
A laminate film with two or more ferromagnetic layers separately arranged on the surface thereof. (planar structure).
The second aspect of the present invention is a magnetic element.
The present inventors carried out extensive studies on a magnetic element consisting of a discontinuous magnetic film and electrodes of a ferromagnetic material, said discontinuous magnetic film being made of a mixture of a dielectric material and a ferromagnetic material, and said electrodes being arranged adjacently to the discontinuous magnetic film through a tunnel barrier. As the result, it was found that it is possible to obtain a more remarkable magnetoresistance effect and to realize a ferromagnetic tunnel effect element with a smaller resistance, if one electrode of ferromagnetic material is laminated on the discontinuous magnetic film, with a dielectric layer interposed between them, and the other electrode of ferromagnetic material is substantially in contact with the ferromagnetic material in the discontinuous magnetic film.
The magnetic element as mentioned above produces the tunnel magnetoresistance due only to the tunnel current across the electrode of ferromagnetic material and the discontinuous magnetic film through the layer of dielectric material. Therefore, it is only necessary to control the thickness of one layer of dielectric material. This permits one to prevent the fluctuation of resistance and magnetic field sensitivity due to variation in tunnel barrier thickness. In addition, the fact that one of the electrodes is substantially in contact with the ferromagnetic material in the discontinuous magnetic film makes it difficult for the ferromagnetic material in the granular magnetic film to undergo spin reversal due to temperature rise or disturbed magnetic field. This means that the magnetic element is stable.
The magnetic element of the present invention may be constructed such that the first and second electrodes of ferromagnetic material are arranged on the layer of dielectric material along the surface of the discontinuous magnetic film. In other words, it is a ferromagnetic tunnel effect element of planar type, which can be manufactured easily.
The magnetic tunnel element according to the first and second aspects of the present invention may be applied to storage systems (such as magnetic memory and magnetic reproducing head for magnetic recording and reproducing units) and magnetic devices (such as magnetic sensor).